1. Field of the Invention
The present invention relates generally to an apparatus and method for allocating resources in a wireless communication system, and in particular, to an apparatus and method for allocating resources in an Orthogonal Frequency Division Multiple Access (OFDMA) mobile communication system
2. Description of the Related Art
Wireless communication systems have been developed for situations where it is not possible to connect fixed wired networks to mobile terminals. Typical wireless communication systems include not only a normal mobile communication system providing voice and data services, but also a Wireless Local Area Network (WLAN), a Wireless Broadband (WiBro) system, a Mobile Ad Hoc network, and the like.
Recently, in wireless communication systems, Orthogonal Frequency Division Multiplexing (OFDM) is under active research and development, and has been put into practice. OFDM, a scheme for transmitting data using multiple carriers, is a kind of Multi Carrier Modulation (MCM) that converts a serial input symbol stream into parallel symbol streams, and modulates each of them with multiple orthogonal sub-carriers, i.e. sub-carrier channels, before transmission.
A wireless communication system employing this multi-carrier transmission scheme was first applied to military radios in the late 1950s, and OFDM, which is the typical multi-carrier transmission scheme for overlapping multiple orthogonal sub-carriers, was developed in the 1970s. OFDM converts a serial input symbol stream into parallel symbol streams, and modulates them with multiple orthogonal sub-carriers before transmission, and the OFDM scheme can be widely applied to digital transmission technologies such as Digital Audio Broadcasting (DAB), Digital Television. Wireless Local Area Network (WLAN), Wireless Asynchronous Transfer Mode (ATM), and the like.
It is known that OFDM, a system suitable for the wireless communication environment where Line of Sight (LOS) is not guaranteed in multiple paths, can provide an efficient platform for high-speed data transmission with its advantage of being robust against multi-path fading. That is, OFDM can efficiently overcome frequency selective fading because it divides the entire channel into multiple orthogonal narrow-band sub-channels before transmission.
Also, OFDM is most effective for high-speed data transmission because it can cancel Inter-Symbol Interference (ISI) by inserting, into a header of a symbol, a periodic Cyclic Prefix (CP) which is longer in length than a delay spread of a channel. Due to these advantages, the IEEE 802.16a standard has been established, and IEEE 802.16a supports a Single-Carrier System. OFDM, and OFDMA.
OFDMA is a multiple access scheme that divides a frequency domain into sub-channels each composed of multiple sub-carriers, divides a time domain into multiple time slots, and then performs resource allocation taking both the time and frequency domains into account by independently allocating the sub-channels to individual users, thereby enabling accommodation of multiple users with the limited frequency resources.
FIG. 1 illustrates exemplary resources in time and frequency domains in a general OFDM wireless communication system.
In the common OFDM system, because it is typical that one modulation symbol (for example, a Quadrature Phase Shift Keying (QPSK) or a 16-ary Quadrature Amplitude Modulation (16 QAM) symbol) is transmitted over one sub-carrier, it can be considered that the sub-carriers are unit resources. In FIG. 1, the horizontal axis indicates the time axis, and the vertical axis indicates the frequency axis. Reference numeral 101 denotes one sub-carrier, and reference numeral 102 denotes one OFDM symbol. Commonly, as shown in FIG. 1, one OFDM symbol 102 is composed of multiple sub-carriers. Also, the common OFDM system groups multiple OFDM symbols as shown by reference numeral 103, and defines each group as a basic transmission unit. In the specification, the basic transmission unit composed of several OFDM symbols will be referred to as a Transmission Time Interval (TTI). Therefore, as shown in FIG. 1, one TTI is composed of multiple OFDM symbols. In addition, it can be noted that if one smallest rectangle shown in FIG. 1 is called a ‘time-frequency bin’, one TTI is composed of multiple time-frequency bins.
In the common OFDM system, it is typical that one TTI is composed of multiple physical channels. The term ‘physical channel’ refers to channels for transmitting different kinds of information, like a paging channel, packet data channel, packet data control channel, reverse scheduling channel, etc., all of which are needed in the common mobile communication system. For example, referring to FIG. 1, in one TTI, some resources, i.e. some time-frequency bins, are used for the paging channel; some resources are used for a common control channel for providing system information; some resources are used for the packet data channel for transmitting user data; and some resources are used for the packet data control channel for transmitting control information used for demodulation of the packet data channel. Although not mentioned above, it should be noted that there are other possible physical channels for other objects.
As described above, the common OFDM wireless communication system has 2-dimensional resources in the time and frequency domains, and the time-frequency 2-dimensional resources can be subdivided into small groups and then allocated to multiple terminals. Because the terminals may be different from each other in terms of the amount of their necessary resources, an efficient agreement on which resource, i.e. time-frequency bin, is allocated to each terminal should be made between a transmitter and a receiver, and the allocated resources could be able to be indicated accordingly. For example, if 5000 bins exist in one TTI as described above, the transmitter should be able to efficiently provide the receiver with information indicating that it has allocated bins #1˜#100 to a first receiver, and bins #101˜#600 to a second receiver. The method of indicating one allocated resource in the manner of indicating which sub-carrier in which OFDM symbol is allocated to a terminal on a sub-carrier by sub-carrier basis, is very inefficient. This is because the conventional method needs too much information to notify a certain terminal of the resource allocated thereto.
To solve this problem, the allocated resources can be indicated with use of a Localized Resources Channel (LRCH) scheme that for 2-dimensional resources, i.e. multiple time-frequency bins, in one TTI, configures a channel by grouping adjacent resources among the 2-dimensional resources in the TTI and indicates the channel, and of a Distributed Resources Channel (DRCH) scheme that configures a channel by grouping resources being spaced according to a particular rule, among the 2-dimensional resources in one TTI, and indicates the channel.
DRCH(N, k) as used herein refers to the resources corresponding to a kth group when time and frequency resources in a TTI are divided into N groups having a distributed or scattered pattern.
FIG. 2 illustrates an example of allocating resources using the DRCH scheme in a general OFDMA system.
Referring to FIG. 2, there are 8 OFDM symbols in one TTI. The OFDM symbols are indicated by L=0 through L=7. One OFDM symbol is composed of 32 sub-carriers. The 32 sub-carriers included in one OFDM symbol are indicated by n=0 through n=31. In FIG. 2, the resources corresponding to DRCH(8, 0) with N=8 and k=0 are shown by the hatched rectangles denoted by reference numeral 200. The resources of DRCH(8, 0) are configured in the following manner.
In each OFDM symbol, 32 sub-carriers are divided into N (N=8 in FIG. 2) groups. Sub-carriers included in each group are characterized in that they are separated by an equal distance in the frequency axis. That is, sub-carriers belonging to a group 0 are sub-carriers corresponding to n={0, 8, 16, 24}; sub-carriers belonging to a group 1 are sub-carriers corresponding to n={1, 9, 17, 25}; sub-carriers belonging to a group 2 are sub-carriers corresponding to n={2, 10, 18, 26}; and sub-carriers belonging to a group 3 are sub-carriers corresponding to n={3, 11, 19, 27}. In addition, sub-carriers belonging to a group 4 are sub-carriers corresponding to n={4, 12, 20, 28}; sub-carriers belonging to a group 5 are sub-carriers corresponding to n={5, 13, 21, 29}; sub-carriers belonging to a group 6 are sub-carriers corresponding to n={6, 14, 22, 30}; and sub-carriers belonging to a group 7 are sub-carriers corresponding to n={7, 15, 23, 31}.
For N=8, sub-carriers included in each OFDM symbol are characterized in that they are separated by an equal distance in the frequency domain. Finally, resources in the frequency and time domains corresponding to DRCH(8, 0) are defined by a unique sequence of each base station. The sequence has as many elements as the number of OFDM symbols included in one TTI. That is, because element positions of DRCH are designated every symbol, the sequence has as many elements as the number of symbols, for example, elements 0, 3 and 1. In the case of FIG. 2, the sequence S={0, 3, 1, 7, 2, 6, 4, 5}. The sequence is an index designating a group in each OFDM symbol.
In other words, in a base station with S={0, 3, 1, 7, 2, 6, 4, 5}, resources in the frequency and time domains corresponding to DRCH(8, 0) are defined as the resources included in DRCH(8, 0) that gathers sub-carriers included in each of a group 0 of a first OFDM symbol, group 3 of a second OFDM symbol, a group 1 of a third OFDM symbol, a group 7 of a fourth OFDM symbol, a group 2 of a fifth OFDM symbol, a group 6 of a sixth OFDM symbol, a group 4 of a seventh OFDM symbol, and a group 5 of an eighth OFDM symbol in the corresponding TTI.
The foregoing can be expressed in a general manner as follows. In the base station with S={0, 3, 1, 7, 2, 6, 4, 5}, resources in the frequency and time domains corresponding to DRCH(8, k) are sub-carriers corresponding to groups expressed as {(0+k) % N, (3+k) % N, (1+k) % N. (7+k) % N, (2+k) % N, (6+k) % N, (4+k) % N, (5+k) % N} in OFDM symbols in the TTI. Here, “%” denotes a modulo operation.
Therefore, it can be noted in FIG. 2 that resources in the frequency and time domains corresponding to DRCH(8, 4) 202 are achieved by gathering sub-carriers included in {4% 8, 7% 8, 5% 8, 11% 8, 6% 8, 10% 8, 8% 8, 9% 8}, i.e. in groups corresponding to {4, 7, 5, 3, 6, 2, 0, 1}, in OFDM symbols in the TTI.
LRCH(N, k), which is another resource allocation unit definition method. refers to the resources corresponding to a kth group when time and frequency resources in a TTI are divided into N groups having a localized pattern.
FIG. 3 illustrates an example of allocating resources using the LRCH scheme in a general OFDMA system.
Referring to FIG. 3, it can be noted that there are 8 OFDM symbols in one TTI, and the OFDM symbols are indicated by L=0 through L=7. One OFDM symbol is composed of 32 sub-carriers. The 32 sub-carriers included in one OFDM symbol are indicated by n=0 through n=31.
In FIG. 3, resources corresponding to LRCH(4, 0) with N=4 and k=0 are shown by reference numeral 300. The 64 sub-carriers with n=0˜7 included in 8 OFDM symbols in the TTI constitute LRCH(4, 0) 300. The 64 sub-carriers with n=8˜15 included in 8 OFDM symbols in the TTI constitute LRCH(4, 1) 302. The 64 sub-carriers with n=16˜23 included in 8 OFDM symbols in the TTI constitute LRCH(4, 2). The 64 sub-carriers with n=24˜31 included in 8 OFDM symbols in the TTI constitute LRCH(4, 3).
The resource indication methods based on DRCH and LRCH can be simultaneously applied for the same time-frequency resources. For example, it is possible to first divide time-frequency resources into a specified number of DRCHs, for allocation, and then re-divide the remaining resources into LRCHs, for allocation. On the contrary, it is also possible to first divide the time-frequency resources into LRCHs, for allocation, and then re-divide the remaining resources into DRCHs, for allocation.
FIGS. 4 and 5 illustrate examples of simultaneously using DRCH and LRCH for the same time-frequency resources.
FIG. 4 illustrates a resource allocation example of configuring LRCH(4, 0) 404, LRCH(4, 1) 406, LRCH(4, 2) 408 and LRCH(4, 3) 410 using the resources left after first allocating DRCH(16, 0) 400 and DRCH(16, 8) 402 in a general OFDMA system.
FIG. 5 illustrates an example of configuring DRCHs 502, 504 and 506 using the resources left after first allocating LRCH(4, 2) 500 in a general OFDMA system.
The foregoing OFDM mobile communication system can allocate information on DRCH and LRCH data channels configured as shown in FIGS. 4 and 5, to terminals over a particular Data Control Channel (DCH) that all terminals receive.